Temperature is a fundamental yet hitherto underappreciated thermodynamic parameter in many environments. In cellular environments temperature may affect reaction kinetics, chemical equilibria, as well as the physical states of nucleic acids and proteins. Quantum dot-based nano thermometers are promising because of their lasting luminescence stability. Previously published ratiometric temperature-sensitive Mn2+-doped quantum dots were made by using expensive and air-sensitive chemicals, and their temperature-sensing capability was not demonstrated in water-based solutions. Furthermore, previously published Mn2+-doped QDs synthesized with air-stable chemicals did not exhibit ratiometric temperature sensitivity.
Generally, QD nano thermometers rely on temperature-dependent changes in their excitonic emission characteristics, a property that extends to the single-particle level. Typically, at higher temperatures, the excitonic emission exhibits red-shifted frequency and increased non-radiative relaxation rates (shortened excited-state lifetime, also manifested as broadened emission profile and reduced intensity). These temperature-dependent properties have been used to detect the temperature of individual living cells (Yang et al., 2010, In The 23rd IEEE Micro Electro Mechanical Systems Conference, Hong Kong, 963-66; Maestro et al., 2010, Nano Lett., 10, 5109-15, which are incorporated herein by reference as if fully set forth). Using QDs as nano thermometers, Yang et al. recently reported the first experimental evidence of heterogeneous intracellular temperature progression responding to the chemical Ca2+ shock and the physical cold shock (rather than the commonly assumed picture that the intracellular temperature is homogeneous under all conditions) (Yang et al., 2011, ACS Nano, 5, 5067-71, which is incorporated herein by reference as if fully set forth). The report also underscores the need for much-improved local temperature reporters because the single-parameter quantum dot temperature sensors (based solely on the emission frequency or intensity, or the excited-state lifetime) are not sufficiently accurate for quantifying the biological and biochemical heat generation. Beyond single-parameter temperature sensing, a ratiometric scheme is expected to be more accurate because it uses two readouts to afford self-calibrated results. Recently, a ratiometric scheme has been shown to report temperature in cellular environments using rare earth-doped structures and semiconducting polymer dots (Ye et al., 2011, J. Am. Chem. Soc., 133, 8146-49, which is incorporated herein by reference as if fully set forth). The first example of using emission intensity ratio (EIR) to sense temperature based on Boltzmann-type distribution was demonstrated in rare earth-doped materials (Kusama et al., 1976, Jpn. J. Appl. Phys., 15, 2349-68, which is incorporated herein by reference as if fully set forth) and subsequently studied extensively within the family of rare earth-doped materials (Berthou and Jorgensen, 1990, Opt. Lett., 15, 1100-02; Wade et al., 2003, J. Appl. Phys., 94, 4743-56, which are incorporated herein by reference as if fully set forth). Recently, Vlaskin et al. extended the EIR technique to sense temperature using Mn2+-doped QDs (Vlaskin et al., 2010, Nano Lett., 10, 3670-74, which is incorporated herein by reference as if fully set forth). The Mn2+-doped QDs have also been shown to exhibit temperature-sensing capability to show changes within 0.2° C. using the relative intensity between the excitonic and the Mn2+ emissions, which are both temperature dependent (Vlaskin et al., 2010, Nano Lett., 10, 3670-74, which is incorporated herein by reference as if fully set forth). The salient properties of the Mn2+-doped QDs make them a possible candidate for a much-improved local temperature sensor for intracellular thermometry. However, the temperature-sensing capability of the Mn2+-doped QDs has previously only been demonstrated in the bio-incompatible toluene solvent.